Electrochemical batteries have for some time been used as a means to power a variety of electronic consumer products. In this regard, conventional batteries are usually of the type having an anode, a cathode, a porous separator to maintain physical separation between the anode and cathode, and a suitable electrolyte supplying a source of positive and negative ions which migrate between the anode and cathode during use.
The separator that is employed in electrochemical batteries should exhibit several desirable functional attributes. For example, the separator must be sufficiently porous or permeable to allow ion migration between the anode and cathode during use. The separator must be sufficiently thin to allow sufficient active material to be placed in the battery to achieve the desired capacity. Finally, from a battery manufacturing process point of view, the separator must have sufficient puncture strength to resist penetration and damage from the electrodes (such electrodes may have rough surfaces), which would lead to internal shorts and high scrap rates in production. This necessary balance of properties (i.e., high porosity and puncture strength in a thin separator) is often difficult to achieve.
In addition, for high energy lithium battery systems, it is also desirous that the separator provide a shutdown function. This is defined as a decrease in porosity to an extent that an uncontrolled reaction of potentially explosive magnitude is prevented from occurring. This should occur at some temperature well below the melting and/or ignition point of lithium. Membranes made of polypropylene (PP) will shutdown, but the melting point of PP (.about.165.degree. C.) is too near the melting point of lithium. Therefore, membranes made of polyethylene (PE) (with a melting point of (.about.135.degree. C.)) are preferred. However, PE membranes tend to have lower puncture strengths than PP membranes of similar thickness.
The use of microporous membranes as battery separators either as single plies or as laminates is well known. In this regard, microporous membranes disclosed in U.S. Pat. Nos. 3,558,764 to Issacson et al, 3,843,761 to Beirnbaum et al., and 3,679,538 to Druin et al (the entire content of each such U.S. Patent being incorporated expressly hereinto by reference) have been commercialized for battery separators and other applications by Hoechst Celanese Corporation under the registered trademark Celgard.RTM.. In general, these patents disclose single ply microporous polyolefin membranes.
U.S. Pat. Nos. 4,650,730 and 4,731,304 each to Lundquist et al (the entire content of each such U.S. Patent being incorporated expressly hereinto by reference) disclose sheet products said to be useful as battery separators which have at least two microporous plies which are coextensively bonded together to form a unitary sheet product. When subjected to elevated temperatures (as when shorting occurs in a battery due to abuse or for other reasons), one of the plies is intended to melt and transform into a non-porous membrane, thus shutting down the current flow and the battery.
Recently, cross-plied microporous membrane separators for use in so-called button cell lithium cathode batteries have been proposed as evidenced in Japanese Patent Application (Kokai) Nos. 59-12559, 63-72063 and 59-173948. Each of these Japanese Patent Publications generally disclose relatively small circular-shaped battery separators for use in button cell batteries, whereby the separators include two microporous membrane plies which maybe laminated to one another such that the axis of one ply is angularly biased (preferably orthogonal) relative to the axis of the other ply. The disclosed process for forming such button cell cross-plied microporous membrane separators generally includes superposing one sheet of microporous membrane over another sheet of microporous membrane so that the axes are angularly biased, and then punching or stamping the circular-shaped separator from the superposed microporous membrane sheets.
The cross-plying of microporous membrane sheets could have benefits for non-button cell batteries in terms of increased strength (puncture resistance). For example, cylindrical or rectangular battery cell constructions requiring a continuous separator sheet whose length is substantially greater than the effective cell diameter (i.e., so that the separator sheet may seamlessly be rolled or generally sinusoidally folded between anode and cathode layers) could benefit from the increased strength and puncture-resistance properties that cross-plied microporous membrane separators may provide. In addition to such increased strength and puncture-resistance properties, at least one of the cross-plied microporous membrane layers could be formed of a relatively low melt-point polymer and thus provide a thermal fuse to stop uncontrolled electrochemical reactions from occurring within the cell.
To date, however, the art has not proposed any means by which a continuous seamless sheet formed of cross-plied microporous membranes could be fabricated. It is towards supplying such a need that the present invention is directed.